This invention relates to video cameras and is more particularly concerned with the problem of shading ears in video cameras.
In video cameras of the type employing photoconductive targets, for example, such as Vidicon, Plumbicon, Saticon, and Newvicon cameras, the accumulation of electric charge at areas of the target beyond the normally scanned area causes an effect known as "shading ears". Since the tube target integrates incident light, high level charges build up and propagate along the target face to the active video area. The high charge also causes beam pulling near the ends of the raster, changing the sweep velocity and increasing the output of the camera tube. The net effect, as seen on a television receiver, is an increase in brightness of the areas at the edges of the video raster, i.e., "shading ears". One method of eliminating shading ears at the top and bottom of the raster is to modify the vertical deflection voltages in order to speed up the sweep just before and just after vertical retrace so as to scan more of the target area vertically (vertical overscan). Although this method works well in the vertical direction, it is very difficult to implement in the horizontal direction, because speeding up the horizontal sweep in a manner similar to the vertical reduces horizontal retrace time and produces large fly-back pulses which are difficult to accommodate. Moreover, the usual horizontal deflection yokes cannot respond to quick changes in the sweep speed because of their inductance.
In U.S. Pat. No. 3,755,707, issued Aug. 28 1973, it is proposed to reduce edge brightening effects in a camera tube of the Vidicon type by increasing the potential applied to the cathode of the electron gun at and near the beginning and end of field and line deflection in the tube in order to increase beam current. The technique proposed in this patent does not accomplish the results of the present invention, however.